Regional Bell operating companies (RBOCs), and telephone companies (TELCOs) in general, employ metallic loop test (MLT) systems for testing metallic wire links of telephone communication systems, e.g. plain old telephone services (POTS). Such metallic loop test systems, standards for which were originally specified by AT&T Technologies, comprise a software-controlled supervisory mechanism, including user interface, line record database, test coordination and interpretation, a central office (CO)-based subscriber loop test system, and a remote measurement unit (RMU).
As diagrammatically illustrated in FIG. 1, the remote measurement unit, shown at 11, is customarily located at a digital loop carrier site 13, where there is access to a metallic link (tip/ring) pair 15, which provides a direct loop connection to a subscriber equipment termination site 17.
Testing of the metallic subscriber loop 15, which typically involves line measurements of DC/AC voltage, DC/AC resistance, line capacitance, etc., is conducted by the remote measurement unit 11 in response to remotely initiated test commands, supplied to the digital loop carrier site via the central office, shown at 10. The MLT-prescribed protocol of a loop test request from the central office 10 includes specifying the test to be conducted, the time required for the line voltage (of loop 15) to settle after connecting the loop to the measurement circuitry of the remote test unit 11, and what loop electrical parameter is to be tested.
When the remote test unit 11 initiates a test of the subscriber loop 15, it connects (via a controlled set of relay contacts, not shown) the metallic link pair 15 to test measurement circuitry 12 within the RMU and waits for a prescribed period of time, that is of sufficient duration for the electrical charges on line 15 to settle. Once the line has settled, testing of the line by loop-connected test measurement circuitry 12 of the RMU is conducted, with monitored line electrical parameters being digitized by an analog-digital converter 23 and stored in attendant RAM of the test unit's microcontroller 25, so that the test results may be reported to the remote MLT-supervisory site. The metallic line is coupled to the input of analog-digital converter 23 through a resistor ladder network 24, respective sections of which are controllably switched into the input path of analog-digital converter 23, in order that its digitized output will appropriately reflect a prescribed range of voltage to be monitored.
The settling time is customarily specified at a fixed interval of sufficient duration to accommodate a worst case condition associated with the longest subscriber loop in the network. Because this worst case condition can be expected to be considerably longer than the settling time of the majority of the subscriber loops, the efficiency of the testing mechanism suffers, as it is necessary to delay testing of a `typical` loop until the fixed, worst case settling time has expired.